For the purposes of the disclosure, the term light refers to any electromagnetic radiation that can be manipulated by optical devices, such as ultraviolet light, visible light, and infrared radiation. The detection axis of a photo-detector can be, for example, the symmetry axis of a field of vision, such as the optical axis of the photo-detector. In terms of geometry, it is a spatial line which begins in that photo-detector. Each photo-detector may be, for example, either end of an optical fiber or bundle, in particular including an optical exciting probe. The optical fiber(s) then lead, for example, to an optoelectronic transformer, such as a spectrometer. A spectrometer includes, for example, an entrance slit, a diffracting element, such as a grating, and an optoelectronic transformer, as well as, optionally, a control unit.
A measuring device of the type described above is known, for example, from DE 195 28 855 A1, the entire disclosure of which is incorporated herein by reference, in particular regarding the detection of the spectral energy distributions Φ and of the luminance factor functions β. The light from the light source scattered on/in the inner layer can exit from the hollow body through the light-exit opening. At a specimen located in front of the light-exit opening, the light is remitted, at least partially, to the light-exit opening (back-scattered and/or reflected there) so that it re-enters the hollow body. A first photo-detector is provided for detecting this light (measurement light, in which properties of the specimen are encoded) that is diffusely reflected into the hollow body; a second photo-detector is provided for the detection of the light that is scattered within the hollow body on/in the diffusely scattering layer (reference light, in which are encoded the properties of the light source). The inclusion of the reference light serves to determine the spectral energy distributions measured in reflection and/or transmission regardless of short-term fluctuations of the wavelength-dependent optical transmission properties of the detection beam path and the emission characteristics of the light source.
Such measuring devices are used, among other applications, in manufacturing and/or in the quality control of optical products. Here, it is often desirable to measure optical properties, for example, reflection and/or transmission behavior, as a function of the wavelength of the light. An example of this is the optical analysis of filter layers acting as an infrared filter, which block heat radiation but should allow visible light to pass as unimpededly as possible. Such filter layers are applied, for example, to architectural glass or automotive glass. Another example are anti-reflective coatings, in particular for broadband antireflection, which should have the lowest possible reflection within the range of visible light. Measurements of their spectral dependence is desired both during the manufacturing process of such coatings and as part of the final quality control of their optical properties.
In order to determine the spectral energy distributions of the specimen surfaces of interest that are measured in reflection and/or transmission regardless of long-term changes in the wavelength-dependent optical transmission properties of the detection beam path and the emission characteristics of the light source, it is desirable to calibrate the measuring device by measuring a reference standard in the detection beam path of the measuring device. As a rule, at least one so-called white standard is used, which scatters incoming light diffusely and typically has a maximum reflectance and/or transmission rate at all wavelengths to be measured at the specimen.
To calibrate the measuring devices known to the art, either the reference standard is moved in front of or instead of the specimen into the detection beam path, or the measuring device is moved in parallel into a calibrating position away from the specimen, such as in DE 195 28 855 A1. In all of these cases, due to the relative movement, inaccuracies occur in the positioning of the object that is being moved. Because of this, the reference standard is not always at the same distance from the light-exit opening as the specimen. Also, the inclination of the reference standard relative to the light-exit opening can vary due to this movement. Minor variations in distance or orientation, however, lead to large differences in the measured light intensities, i.e., result in a large error of the calibration measurement. A large error in the calibration is then continued into the specimen measurements.
Also, the relative movement may cause a disturbance of the measuring geometry. For example, the optical properties of optical fibers change when they are moved. In addition, the reference standard in the measuring devices known to the art can become soiled. This can happen in particular due to the monitored manufacturing process, for example, by sputtering. Also, the ambient temperature can affect the reference standard in the measurement devices known to the art. The accuracy of the calibration can be significantly affected. In addition, the handling of the reference standard is quite complex if the specimen is used and measured in a vacuum.